Since its introduction in 19751, suppressed conductometric Ion Chromatography (IC) has emerged as the preferred method for performing ionic analysis. The worldwide annual market today of IC equipment and related consumables is estimated to be of the order of US $200 million. There has been interest in carrying out IC in a miniature scale since the early years. As early as 1983, suppressed capillary IC was reported.2 The entire system was not miniaturized. Subsequently, more refined versions of suppressed conductometric capillary IC instrumentation were presented, including a portable briefcase-sized unit.3 The use of small tubular membrane-based continuous ion exchangers in these units is directly traceable to the earlier deployment of essentially identical devices for use in capillary electrophoresis (CE).4 Other capillary-based efforts include internally latex-coated fused silica columns that could be used at elevated temperatures;5 however, only optical detection was possible. More recently a similar system has been described in which separation takes place in a latex-coated channel microfabricated on silicon (as well as in non-negligible lengths of connecting tubing) with off-line optical detection.6 Nevertheless, there is a belief, perhaps for good reasons,7 that planar devices, however fabricated, are going to be the key devices of the future. In this view, Kang et al.8 have described a multiple parallel channel microfabricated device for conducting IC. The stationary phase was prepared in situ and each channel contained its own dedicated conductivity detection electrodes. The limit of detection (LOD) of injected analytes was stated to be about 50 mM, which may have been well above the concentration (unspecified) of the biphthalate or carbonate eluent used. This performance level underscores the importance of the need to develop some means of carrying out continuous ion exchange based suppression for use in a planar format.
Synthetic polymer based ion exchange membranes have largely constituted the foundation of macro- and micro-scale conductivity suppression over the last two decades. However, it will be difficult to mass fabricate low-cost devices incorporating such membranes due to sealing and fabrication issues. This focuses on an alternative approach that relies on continuously flowing liquid membranes and draws on the recent teachings of Yager et al. and Kitamori et al. as detailed below.